1. Field of the Invention
The present invention relates generally to the construction and operation of plasma etching systems, and more particularly to methods and apparatus for powering single wafer plasma etch systems including a pair of parallel, opposed electrodes.
Single wafer, parallel plate plasma etching systems comprise a lower electrode, generally referred to as a chuck electrode, and an opposed upper electrode, generally referred to as a counter electrode. Either electrode may be powered by a radio frequency signal while the other electrode is grounded to induce the plasma necessary to accomplish the etching.
In order to enhance the etch rate afforded by a plasma etching system, it is desirable to induce a high voltage across the electrodes, imparting very high energy levels to the system. The voltage which can be applied, however, is limited by the occurrence of stray discharges and arcing between the driven electrode and the grounded reactor housing. Such discharges dissipate RF power, can damage the equipment, and most importantly cause discontinuities and instabilities in the plasma which can cause non-uniformity in the etch rate across the wafer.
It would therefore be desirable to provide apparatus and methods for performing plasma etching at relatively high voltage and power levels without the occurrence of arcing and stray discharges as have just been described.
2. Description of the Background Art
U.S. Pat. No. 4,626,312 to Tracy proposes that stray electrical discharges in parallel plate plasma reactors can be reduced by dividing the applied voltage between the upper and lower electrodes. Two specific systems for achieving such voltage division are described. The first system is utilized in low frequency reactors (400 kHz) and employs an ungrounded RF generator to feed the electrodes in a grounded reactor chamber. It is apparently assumed that the voltage applied across the electrodes will float approximately equally about the ground potential of the reactor vessel, thus minimizing the potential difference between either electrode and ground. While generally achievable, the desired equal division of the voltage between the electrodes cannot be fully realized because of asymmetries in the construction of the reactor vessel, electrodes, and electrical feed lines, as well as the unbalanced impedance load placed on the counter electrode by the wafer. Thus, a precise division of the applied voltage cannot be effected and the theoretical maximum voltage cannot be utilized.
The second system is intended primarily for high frequency (13 MHz) plasma etch reactors, and includes a grounded RF generator connected across the opposed electrodes with a variable inductor between the chuck electrode and ground. The inductor causes a phase shift in the voltage on the chuck electrode which, together with the phase shift resulting from the capacitive nature of the electrodes themselves, results in a phase shift between the voltages on the two electrodes which approaches 180.degree.. As can be seen in FIG. 3 of the patent, however, the phase shift will not reach 180.degree. so that the maximum potential difference across the plasma without arcing cannot be achieved.
See also, U.S. Pat. Nos. 4,399,016; 4,253,907; and 4,134,817, which disclose alternate systems for powering parallel plate plasma reactors.
For these reasons, it would be desirable to provide a system and method for dividing voltage between the upper and lower electrodes in a plasma etch system in a highly controlled manner to maximize the potential difference between the electrodes which can be employed to induce a plasma without causing stray discharges between either of the electrodes and the reactor chamber.